Spade drills are rotary cutting tools having one or more cutting edges. A spade drill generally includes a spade drill insert secured in a holder, but may also be manufactured in one piece. Spade drills including a spade drill insert and a holder are most widely used for drilling holes having diameters of 1 to 6 inches. Spade drills may be used for drilling metal work pieces, as well as work pieces of other materials, such as wood and plastics. Spade drills and certain other cutting tools include chip control geometry adjacent to the cutting edge. This chip control geometry improves cutting performance during cutting operations that produce chips. The chips are formed during metal cutting by the process shown in FIG. 1. The cutting edge 13 of the cutting tool 10 moves into the work piece 12 in the general direction of the arrow shown in the FIG. 1. Chip 11 is formed from the work piece 12 leaving a thickness 18. The relative motion between the cutting tool 10 and the work piece 12 during cutting compresses the work piece material in the area 14 in front of the cutting tool 10 and induces primary or shear deformation of the work piece which begins to form the chip 11. The chip 11 then passes over the rake face 16 of the cutting tool 10 and undergoes secondary deformation due to the shearing and sliding of the chip 11 against the tool 10. The chip 11 subsequently breaks away from the work piece 12 to complete chip formation.
The physical properties of the material to be cut and the parameters of the cutting operation, including feed rate, cutting speed, depth of cut, rake angle, tool nose radius, lead angle, primarily control chip formation. Chips may be formed in a variety of shapes, from long, continuous metal strips, which may be severely deformed or in the form of long curls, to small fragments. The properties of the material that influence chip formation include yield strength, shear strength under compressive loading, hardness, ductility, as well as other properties. For example, cutting highly ductile materials may involve extensive plastic deformation of the chips, resulting in long, continuous chips. Longer chips remain in contact with the tool face longer, causing frictional heat buildup and thermal stress on the cutting edge. Long continuous chips are also more difficult to discharge from the cut in the work piece, especially during cutting operations such as boring or drilling hobs.
Cutting parameters that influence chip formation include lead angle, cutting edge geometry, feed rate, cutting speed, and depth of cut. These parameters may be controlled by the machinist in order to influence chip formation. Chips may separate from the work piece in one of three basic ways: they break off by themselves; they break against the cutting tool; or they break against the work piece. Machinists attempt to balance the foregoing cutting parameters to produce chips that are short and self-breaking. Chips of that type are easily discharged during the cutting operation and do not damage either the work piece or the cutting tool.
Certain materials are more likely to form undesirable chip shapes during cutting. Stainless steel, for example, tends to produce continuous, long, curled chips that may cause chip jamming and increased power consumption. Accordingly, a machinist's control of the parameters affecting chip formation is a particular importance when cutting these materials.
As shown in FIG. 2, conventional two-piece spade drills generally comprise a holder 21 having a clamping slot 24 and a plate-shaped drilling insert 22 which may be secured in the clamping slot 24. The spade drill insert 22 is secured against limbs 27 and 28 of the clamping slot 24 by means of at least one threaded pin 23. The head of the threaded pin 23 may engage a beveled bore 25 of the spade drill insert 22 and is secured in a threaded bore 30 in a limb 27 or 28 of the holder 21. The spade drill insert 22 may be provided with a centering slot 29 or a tab which meshes with a corresponding element of the holder 21 to ensure that the spade drill insert 22 is centered along the axis of rotation 26 of the holder 21.
FIGS. 3(a) and 3(b) depict the conventional spade drill insert 22 of FIG. 2. The spade drill insert 22 is generally plate-shaped and includes a pair of cutting edges 31. The cutting edges 31 extend radially outward from the central axis 26 of the spade drill insert 22 and are separated by 180.degree. about the central axis 26. As noted above, two-piece spade drills are most widely used for drilling relatively large holes, in the range of 1 to 6 inches in diameter. One-piece spade drills, which combine the shank and the cutting edges together in one piece, are typically used for drilling holes of smaller diameters.
There are several advantages to using a spade drills instead of a conventional twist drill to provide a bore in a work piece. Spade drills have heavier cross-sections than comparable twist drills. The additional strength this provides is concentrated along a line from the cutting point to the shank of the spade drill and gives the spade drill greater resistance to end thrusts experienced during piercing of the work piece. The additional strength also gives the spade drill a greater ability to withstand the high torque experienced during rotational cutting of the work piece, and minimizes vibration, chipping of the cutting edges, and drill breakage. Additionally, standard twist drills are likely to wear into a forward taper, which also has the tendency to cause binding. The shorter cutting edges of spade drills, which incorporate a greater back taper, reduce the tendency to bind.
Once worn, the spade drill insert of a two-piece spade drill can be replaced while the holder remains on the machine tool without the necessity to reset stops, break down setups, or increase or decrease the length of a drilling setup. Spade drills also may be more easily preset for use on automatic and computer numerical control machine tools than conventional twist drills.
Spade drills, however, also have certain limitations. As with all material removal operations, chip breaking and chip formation control are significant factors in the efficiency of the cutting operation. As seen in FIGS. 3(a) and 3(b), a conventional spade drill has primary cutting edge 31 with its corresponding rake face 32 for primary material removal from the work piece. A conventional spade drill does not incorporate any chip control geometry on the rake face 32. The conventional design typically produces chips that are as wide as the cutting edges and, therefore, makes chip length control difficult. The large chips may accumulate in the bore being formed and cause jamming of the cutting tool in the work piece, increasing power consumption and resulting in poor drilling tolerances and excessive wear of the cutting tool.
Attempts have been made to add chip formation control features to the cutting edges of spade drill inserts. FIG. 3(c) illustrates an end view of a spade drill 33 modified to include slit-type nick grooves 35 in the cutting edge 36 of the spade drill blade. These slit-type nick grooves 35 prevent the formation of chips that are as wide as the cutting edge. Instead, if the cutting operation parameters are set properly, the chips produced are only as wide as the distance 37 between the slit-type nick grooves 35 because the chips are formed by the several cutting edges 36 between the nick grooves 35. Although the chips produced by spade drill 33 of FIG. 3(c) are smaller in width than those produced by spade drill 22 of FIGS. 3(a) and 3(b), the chips may also be disadvantageously increased in length. That result occurs because, in general, thin chips are more likely to deform and less likely to break when they contact the wall of the bore in the work piece or when they contact the cutting tool. The long chips produced by spade drill 33 may also cling to the drill resulting in a further a reduction in chip formation control.
When operating a conventional spade drill, the drilled bore may quickly become loaded with the chips. Excessive chip loading may cause premature wear and breakage of the drill, particularly when drilling deep bores. Due to the lack of chip control and the corresponding formation and accumulation of large chips, drilling with conventional spade drills requires higher torque and thrust forces than drilling similarly sized bores with other drilling tools, such as twist drills. The long chips that are formed during piercing of the work piece remain in the bore and tend to cling to the cutting tool and jam between the cutting tool and the work piece, thereby causing increased radial forces, tolerance problems, and increased power consumption.
Thus, a need exists for a cutting tool having improved cutting performance, including improved chip control and chip breaking control, and which has a reduced tendency to jam within the hole in the work piece. The need also exists for a cutting tool having improved chip formation control and chip breaking geometry and that will generate chips of an advantageous size and shape under a wide range of cutting parameters when used to cut a variety of materials.